Recently we have shown that homolytic cleavage of the Co-C bond occurs when ethanolamine binds to ethanolamine ammonia-lyase. The rate of homolytic cleavage is consistent with the rate of catalysis. Studies on the substrate-specificity of ethanolamine ammonia-lyase are in progress. Ethoxyethylamine has been particularly useful in demonstrating the lack of specificity of hydrogen transfer and the lack of stereoselectivity of this enzyme. The kinetics and mechanism of cobalamin-dependent methyl-transfer to mercuric ion to give methylmercury as the major product are now completely studied. Electrophilic attack by mercuric ion on a carbanion stabilized by the cobalt atom is a first order process. Similar studies are in progress for methyl-transfer to tin, thallium, gold, platinum, arsenic, selenium, tellurium and sulfur. High resolution NMR and stopped-flow kinetics provide the major techniques to study these reactions. The mechanism of cobalamin-binding to Intrinsic Factor involves the displacement of benzimidazole and coordination of a histidine residue to the cobalt atom. B12 binds in a cleft of the Intrinsic Factor no more than 5 angstrom units from the surface. This distance was determined by using B12-affinity column chromatography. Similar studies on cobalamin- binding to the trans-cobalamins are in progress.